1. Field of the Invention
The present invention is related to plasma processing of semiconductor wafers during the manufacture of semiconductor wafers and, more particularly, to an apparatus and a method for producing a uniform negative ion plasma for plasma processing.
2. Background Description
U.S. patent application Ser. No. 08/595,438 entitled "Negative Ion Inductive Source For Etching High Aspect Ratio Structures" to John H. Keller, teaches a method and apparatus for forming high aspect ratio structures in a semiconductor wafer wherein a series of magnets is disposed between hot and cold plasma regions. A magnetic field from the series of magnets is used to reduce hot electron diffusion, thereby producing a uniform negative ion plasma. The negative ion plasma is used to etch the workpiece.
Reactive Ion Etch (RIE), such as taught in Keller, is used to form shapes on semiconductor wafers. In a typical RIE process, radio frequency (RF) or microwave power is used to excite a gas to form a plasma. The plasma facilitates etching the shapes into the wafer. A typical use of a RIE process is in etching very fine holes, typically minimum dimension sized holes, through a layer, e.g., an insulating layer, to an underlying layer, commonly referred to as "opening contacts" or "opening vias" through the layer. RIE is also used to open minimum width trenches through one or more wafer layers. Typically, these minimum dimension shapes are deeper than at least one of their surface dimensions, and so, have a high aspect ratio.
Charging effects from ionic charging, known as aspect ratio charging effects, occur at the bottom of these high aspect ratio structures, especially, if some part of the structure is an insulator. These aspect ratio charging effects include oxide damage, device damage, threshold voltage shifts, polysilicon notching and reduction of ion current at the bottom of trenches and at the bottom of vias that impedes or even stops etching (known as RIE lag).
Thus there is a need to eliminate aspect ratio charging effects.